docs(angular): add let content (#6393)

src/data/roadmaps/angular/content/let@AwOM0ucg6W7TohdUd7KWT.md

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 # @let
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+@let allows you to define a local variable and re-use it across the template.
+
+Visit the following resources to learn more:
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+- [@official@Angular Official Docs - @let](https://angular.dev/api/core/@let)
+- [@article@Angular's next feature let syntax](https://nhannguyendevjs.medium.com/angulars-next-feature-let-syntax-afba6354112b)